1. Field of the Invention
The present invention relates to ionically conducting membranes and in particular to membrane and electrode assemblies for use in electrochemical cells, and in electrochemical cell stacks.
2. Description of the Related Art
Central to the operation of any electrochemical cell, such as a fuel cell or an electrolytic cell, is the occurrence of oxidation and reduction reactions that produce or consume electrons. These reactions take place at catalyzed electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load (in the case of a fuel cell) or to an external voltage source (in the case of an electrolytic cell), and electrons transfer electric charge between the anode and the cathode through the external circuit. To complete the electrical circuit through the cell, an additional mechanism must exist for internal charge transfer. One or more electrolytes, usually in the form of a solution or ion exchange membranes, provide internal charge transfer by ionic conduction. These same electrolytes must be poor electronic conductors to prevent internal short-circuiting of the cell.
Polymeric ion exchange membranes comprise a broad category of thin solid polymer electrolyte materials that can be used when solvated in electrochemical cells to facilitate ionic conduction between an anode and cathode within an electrochemical cell. Cation exchange membranes allow the transport of cations through membranes, while anion exchange membranes allow the transport of anions through membranes. Cation exchange membranes are most suitable for use in electrochemical cells in either the proton (or acid) form, or in the alkali metal cation (or salt) form. When a solid polymer electrolyte membrane is in the proton, or acid, form the surface and bulk of the membrane, when in contact with pure water has a pH of about one. However, when a solid polymer electrolyte membrane is in the alkali metal cation, or salt, form, the surface and bulk of the membrane when in contact with pure water has a pH of about seven, that is a neutral pH value.
Proton exchange membranes (PEM's) are one category of cation exchange membranes or solid polymer electrolytes that are particularly suitable for use in conjunction with electrochemical cells. PEM's typically have a polymer matrix with functional groups attached that are capable of exchanging cations or protons. The polymer matrix in one instance can consist of an organic polymer such as polystyrene, or other polytetrafluoroethylene (PTFE) analog. In general, the PEM material is an acid with sulfonic acid groups, carboxylic acid groups, or other acidic functional groups incorporated into the organic polymer chains that make up the polymer matrix.
The apparent advantages of using ion exchange membranes in electrochemical cells are numerous. The solid electrolyte membranes are simpler to use and more compact than other types of electrolytes, e.g., aqueous solutions, molten salts, etc. In addition, the use of an ion exchange membrane instead of a liquid electrolyte offers several advantages, such as simplified fluid management and elimination of the potential of corrosive liquids. In systems using an ion exchange membrane, the membrane also serves as an electronically insulating separator between the anode and cathode of an electrochemical cell.
Conventional cation conducting membranes for use in electrochemical cells consist of homogeneous polymer materials. The chemical formula for exemplary cation exchange polymers used to form polymer electrolyte membranes for use in electrochemical cells are presented in FIG. 1. These polymers represent a class of compounds known as perfluorosulfonic acids (PFSA), which are fully fluorinated, i.e., fluorine atoms have replaced all of the sites usually occupied by hydrogen atoms in a hydrocarbon polymer. This makes the polymers extremely resistant to chemical attack. These perfluorosulfonic acid (PFSA) polymers are used in proton exchange membranes such as DuPont's NAFION® 115, where n˜6.5 and m=1, and the Dow Chemical membrane, where n·6 and m=0.
As shown in FIG. 2, PFSA polymers are generally synthesized by the copolymerization of a derivatized, or active, co-monomer with tetrafluoroethylene, (TFE). The sulfonate functionalities (R—SO3−) provide a stationary counter charge for mobile cations (H+, Li+, Na+, etc.), which are generally monovalent. The mobile cations are conducted through the membrane material from one sulfonate group to the next sulfonate group, as illustrated in FIG. 3, which is a schematic drawing of the commonly proposed structure for perfluorosulfonic acid (PFSA) polymers, as typified by NAFION® (a registered trademark of Dupont of Wilmington, Del.).
A sheet of conventional polymer material in the non-ionic sulfonyl fluoride form (formulated as R—SO2F) is first hydrolyzed with a strong base solution, such as NaOH, to convert the polymer to the ionomeric form (see FIG. 2), and then treated with a strong acid solution to convert the polymer to the acid form. The significant reactions are illustrated by equations (1) and (2).R—SO2F+2NaOH→R—SO3Na+NaF+H2O  (1)R—SO3Na+H2SO4→R—SO3H+NaHSO4  (2)
In an electrochemical cell, the cation exchange membrane polymer electrolyte is typically used in the acid, or proton, form, exhibiting a strongly acidic surface when water, or a mixed aqueous/organic solvent (e.g., water-methanol), or an organic solvent (e.g., methanol) is introduced into the electrochemical cell and makes contact with the proton exchange membrane (PEM). On applying a DC electrical potential across the cell, which causes a DC current to flow through the cell, the protons are driven from the PEM/anode interface through the PEM toward the PEM/cathode interface. The anode electrocatalysts and cathode electrocatalysts are in intimate contact with the proton exchange membrane and, in many cases, the electrocatalysts are deposited directly upon the proton exchange membrane. This intimate contact exposes the anodic and cathodic electrocatalysts to the acidic surfaces of the hydrated proton exchange membrane in the acidic proton form. For some electrocatalyst materials, such as some elemental metals, metal alloys, metal oxides, metal borides, metal nitrides, and metal carbides, whether supported or unsupported on a catalyst substrate, this intimate contact can give rise to situations where the electrocatalyst materials, and/or their supports are chemically unstable, and/or they become inactive from an electrocatalytic perspective. Chemical instability, or loss of electrocatalytic activity, of electrocatalyst materials physically in contact with solvated proton exchange membranes arises from chemical reactions between the electrocatalyst materials and the solvent, which is usually water, in a strong acid environment in the absence of cathodic or anodic protection associated with a suitably applied DC cell voltage. These chemical reactions can include electrocatalyst dissolution processes, electrocatalyst dissolution/reprecipitation processes, electrocatalyst corrosion processes, film formation processes, electrocatalyst decomposition processes, etc. The rates of reaction of many of these processes are accelerated considerably in the strongly acidic environment of a hydrated proton exchange membrane.
For any given electrocatalyst material, whether supported or unsupported on an electrocatalyst support material, which is in intimate contact with a solvated (usually hydrated) proton exchange membrane, the nature and magnitude of the electrode potential applied across the electrocatalyst/hydrated proton exchange membrane interface controls the rate of the electrocatalyst degradation processes outlined above. An electrode potential across an electrocatalyst/hydrated proton exchange membrane interface is created when a voltage from a DC power supply is applied to an electrochemical cell comprising a cathode electrocatalyst/hydrated proton exchange membrane/anode electrocatalyst combination, where each electrocatalyst component is in intimate contact with opposing sides of the hydrated proton exchange membrane. This causes a DC current to flow in the circuit that includes the electrochemical cell and the DC power supply. To support the flow of DC current an anodic electrochemical oxidation reaction (e.g., oxygen and/or ozone evolution) takes place at the anode electrocatalyst/hydrated proton exchange membrane interface and a cathodic electrochemical reduction reaction (e.g., hydrogen evolution) takes place at the cathode electrocatalyst/hydrated proton exchange membrane interface. Thus, the nature of the electrode potential (anodic or cathodic) and the magnitude of the electrode potential whether in the hydrogen evolution region, that is, whether it is less than 0.000V (SHE; standard hydrogen electrode), in the region of electrochemical stability of water, that is, between 0.000 V (SHE) and 1.23 V (SHE), or in the oxygen/ozone evolution region, that is, greater than 1.23 V (SHE) have a dominant influence on the degradation processes taking place at electrocatalyst/hydrated proton exchange membrane interfaces.
For purposes of illustration, the influence of the nature and magnitude of the electrode potential applied across an electrocatalyst/hydrated proton exchange membrane interface will be demonstrated for elemental copper, which is known to be a hydrogen evolution electrocatalyst, using the electrode potential—pH equilibrium diagram for the system copper-water at 25° C. in aqueous solutions (FIG. 4) and for a metal oxide, lead dioxide (PbO2), which is known to be an ozone evolution electrocatalyst, using the electrode potential—pH equilibrium diagram for the system lead-water at 25° C. (FIG. 5). In both Figures, for any solution pH, the electrode potential domain between the dashed lines indicated by ‘a’ and ‘b’ corresponds to those potentials where water is electrochemically stable. Below the dashed line indicated by ‘a’, hydrogen evolution from water can take place. However, the magnitude of the cathodic, or negative, electrode potential necessary to be applied to induce hydrogen evolution depends on the electrocatalytic activity of the particular electrocatalyst used. Similarly, above the dashed line indicated by ‘b’, oxygen evolution from water can take place. However, the magnitude of the anodic, or positive, electrode potential necessary to be applied to induce oxygen evolution depends on the electrocatalytic activity of the particular electrocatalyst used.
Typically, hydrated proton exchange membranes, that is, in the acidic proton form, such as NAFION® supplied by Du Pont, Wilmington, Del., have an effective pH of about 1. It can be seen from FIG. 4 that copper used as an electrocatalyst in any acidic aqueous environment having a pH of about 1 is only stable under cathodic electrode potentials corresponding to hydrogen evolution. At potentials more positive than approximately 0.000 V (SHE), copper metal dissolves to give Cu+ and Cu++ cations.
It is well known that an anodic, or positive, electrode potential of at least 1.500 V (SHE) must be applied to an electrocatalyst/aqueous solution interface, where the aqueous solution or environment has a pH of about 1, to evolve ozone gas from water. It can be seen from FIG. 5 that lead dioxide (PbO2) used as an electrocatalyst in any acidic aqueous environment having a pH of about 1 is only stable under anodic electrode potentials corresponding to ozone evolution. At potentials more negative than approximately 1.500 V (SHE), lead dioxide (PbO2) dissolves to give Pb2+ cations. However, at potentials more negative than approximately −0.350 V (SHE), lead dioxide (PbO2) is electrochemically reduced directly to lead (Pb) metal, which is then stable under these negative electrode potentials.
It is clear from these illustrations that for an electrochemical cell comprising copper cathodic electrocatalyst/hydrated proton exchange membrane in the proton form/lead dioxide anodic electrocatalyst, both electrocatalysts will only be stable and useful when a sufficient cell voltage is applied across the cell to give an anodic electrode potential more positive than about 1.500 V (SHE) at the lead dioxide/hydrated proton exchange membrane interface and a cathodic electrode potential more negative than about 0.000 V (SHE) at the copper/hydrated proton exchange membrane interface. However, for this electrochemical cell, and some other related electrochemical cells having a hydrated proton exchange membrane in the acidic proton form, problems arise when, for example, a DC voltage of sufficient magnitude from a DC power supply is not being applied to the cell. This situation arises, for instance, when the cell is being shipped or stored while not in use. While this may not be a problem for many electrocatalysts, other electrocatalysts degrade, dissolve, corrode, or otherwise become inactive when exposed to the acidic surface of the membrane in the absence of a suitably applied electrode potential, such as during shipping or storage of the electrochemical cell. For example, in an electrochemical cell that produces ozone, lead dioxide (PbO2) is a preferred electrocatalyst for the anode electrode and platinum either in the form of unsupported platinum black or as finely divided platinum supported on any electronically conducting, high surface area support, e.g., carbon or graphite powder, is a preferred electrocatalyst for the cathode electrode. When the lead dioxide (PbO2) is left exposed to the acidic surface of the hydrated proton exchange membrane in the acidic proton form in the absence of a suitably applied electrode potential during shipping and storage, the lead dioxide electrocatalyst degrades, dissolves, or is otherwise deactivated as described above, thereby losing some, or all, of its ability to produce ozone from water.
Therefore, a method for storing membrane and electrode assemblies or fully assembled electrochemical cells, or cell stacks, that protect the electrocatalysts from the acidic surfaces of PEMs during shipping and storage would be desirable. It would also be desirable if the method could be applied to electrochemical cells, or electrochemical cell stacks, that are being taken out of service and stored for a period. Furthermore, it would be desirable if the method protected the PEM itself from damage, which may occur if the PEM is allowed to dry out after assembly.